While the earliest slab pots were boxes, it wasn’t long before I moved to an “envelope” format: two slabs plus a bottom.  My basic approach was to make a loaf of clay in about the shape I was shooting for, then to cut the loaf into slabs with a wire.  The loaves were cut the long way, with the wire held about 1/4 inch off the table the loaf sat on.  I usually made three pairs of slabs which would turn into three vases of very similar size and shape.  Bottoms were added as soon as the slabs had dried enough to be turned upside down.

 

Two-slab vase 13″ high 1980

Side view of two-slab vase